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The city’s new electricity supplier, Integrys Energy Services, started alerting Chicagoans on Thursday that it will provide service if they don’t opt out of the plan by Jan. 9.

Those who opt out will remain with ComEd or their current electricity supplier.

On Dec. 7, the city selected Integrys to replace ComEd as electricity supplier for 2.5 million Chicago customers, saying it will save residential households and small businesses roughly 22 percent off their first bills starting in March. City officials say they expect a total savings of $135 to $165 for the average single-family household over the first 15 months of Chicago-based Integrys’ contract.

Chicago’s agreement with Integrys calls for cleaner energy, setting what the city calls a new standard by completely eliminating coal from the fuel mix. Coal currently comprises half of the city’s energy portfolio.

With Integrys, the savings will be the main difference Chicagoans see in their bills, as ComEd will still be responsible for issuing those bills, accepting payments and offering bill payment options, delivering electricity, maintaining electricity lines and meters, and responding to outages, according to the city.

Low-income energy assistance programs such as LIHEAP and PIPP will be unaffected.

Those choosing to enroll with Integrys need take no action. Enrollment will be automatic. But for those Chicagoans choosing to remain with ComEd, Thursday’s mailing provides instructions on how to opt out of the new program — via phone, 1-888-802-2885; or online, at www.integrysenergy.com/il-chicago; or by mail, returning the card contained in the letter — before Jan. 9.

Once enrolled, Chicagoans may leave the program at any time at no cost.

Mayor Rahm Emanuel’s administration said it chose Integrys to serve as the lone supplier because it offered the lowest price margin. Integrys was one of eight companies to respond to the city’s request for qualifications.

It owns six regulated utilities providing natural gas and electricity services, including Peoples Gas and North Shore Gas.

The city’s agreement locked in a rate of 5.42 cents per kilowatt hour until May 2014, with Integrys’ rate for the contract’s final 12 months yet to be determined, though it will be required to always beat or match ComEd’s prices.